Deposition of metals



p 1962 v. E. MEDINA 3,053,741

DEPOSITION OF METALS Filed April 6, 1961 4 6 5/0 /2 /4/6/ZZZZ a2. (Wm/any INVENTOR VZaZvr'ZT/Yedzm ATTORNEYS This invention relates to the electrodeposition of palladium-silver alloys. More particularly, the invention relates to the electrodeposition of a non-porous film of palladium-silver alloy from an ammoniacal solution of the nitrate.

1n the prior ant, electrodeposition is used extensively in industry as a means of putting down a decorative finish on a metal, as well as to supply a surface which is highly protective against destructive atmospheres. More recently, electrodeposition has been used to lay down a highly conductive surface of copper, silver or gold onto metals such as steel or the like which are not good electrical conductors. However, up until now, no convenient and economical method of depositing a non-porous film of palladium-silver alloy onto a metallic or plastic surface has been known.

Accordingly, it is an object of this invention to provide a method of depositing a non-porous film of palladiumsilver alloy.

It is another object of this invention to deposit a nonporous film of palladium-silver alloy in controlled ratios.

These and other objects of the invention will become more apparent from the following detailed description with particular emphasis upon the illustrative examples.

Briefly, the objects of this invention are accomplished by preparing an ammoniacal nitrate solution of palladium and silver, placing the sample to be coated in the solution and applying an electric current across the solution to carry out the deposition. In order to obtain good adhesion of the palladium alloy film onto the sample, it is necessary to chemically clean the surface of the article to be plated order that it is free from surface oxides, etc. Additionally, the surface of the sample must be free from oils, greases and similar substances. This is accomplished by methods known in the art.

The nitrate solutions employed are conveniently prepared by dissolving a palladium-silver alloy in nitric acid and then rendering the solutions basic, i.e., to a pH of from about 75-11, by adding ammonia to the solution. Alternatively, the solution can be prepared by admixing nitrates of silver and palladium and thereafter rendering the solutions ammoniacal. The concentration of the solution is not particularly critical, however, it has been found that solutions containing from 0.5 to about grams of the metal or metals per liter of solution give good results, although solutions containing up to about 150 grams and higher of the metals per liter are conveniently used.

The plating process is carried out at temperatures ranging from about 3590 C. with the preferred temperature range being in the neighborhood of 50-60 C. The current density and voltage of the deposition determine the composition of the deposits. It has been observed, surprisingly, that the composition of the film deposited is not dependent upon the concentration of the palladium-silver alloy solution, but is a function of the current density of deposition. Thus, starting out with a solution of given concentration the composition of the film deposited is varied by changing the current density. This is apparent from the drawing where current density is plotted against the composition of the deposited film, whereon a solution is Pd+Ag The experiments conducted to produce the graph were conducted at a pH of 8-9, a temperature of 55-60 C., and wherein the palladium in the solution amounted to 1.64 grams per liter. From the graph, it is seen that a current density of 10 will deposit a film having a 46% palladium and 54% silver composition, whereas at a current density of 15 ma./cm. the film deposited has a composition of 71% palladium and 29% silver. In order to obtain an even coating upon the sample to be coated, the process is carried out at a current density of from about 1 to 100 ma./cm. and at a voltage of from about 0.5 to 7 volts. However, these constants are varied depending upon the end result desired.

In depositing a film of palladium and silver alloy it is necessary to adjust the proper amount of silver nitrate and palladium nitrate admixed, or a finished alloy of the desired ratio can be dissolved in nitric acid and ammonia added to obtain the proper percentages of the two materials. It has been found that upon completion of the electrodeposition, a uniform non-porous film of the alloy is obtained. Non-porous films have been deposited having thicknesses ranging from about 5 to 100 microns. If the coating is less than about 5 microns, the films were found to be porous.

In the instant electrodepositi-on process, various anodes can be used, however, platinum anodes are particularly satisfactory. Other operable anodes are electrodes made of silver-palladium alloy or stainless steel.

Having described the invention in general terms, the following examples are set forth .to illustrate the preferred embodiment of the invention. Parts are by Weight unless otherwise specified.

Example 1 This example illustrates the uniform coating of a silverized porous polyethylene plastic sheet with a nonporous film of a palladium-silver alloy. The silverized polyethylene sheet was prepared by immersing a 20 cm. square polyethylene porous plastic sheet 5 mils thick having a porosity of 80% and having of the pores in the range of from one to about five microns in a 5% aqueous potassium hydroxide solution and agitating for one minute. The sample is washed in distilled water, and thereafter immersed, with agitation, for one minute in a sensitized solution composed of grams stannous chloride, 500 ml. concentrated hydrochloric acid and 4000 ml. of water. The sample is again washed in distilled water.

The sensitized polyethylene sample is placed in a fiat bottom, glass container only slightly larger than the polyethylene sample. The polyethylene sample is spread flat and attached to the bottom of the container by taping so that the surface of the plastic to be silvered is in a horizontal plane and faces upward. Preferably, the sample is spread in a fixture so that the surface of the sample is elevated A; to inch from the bottom of the container, thus, any sludge produced during the operation will tend to accumulate at the bottom of the bath container rather than on the surface of the sample.

Approximately six ml. of a silver solution per square centimeter of the sample is placed in the bath container. (The silverizing solution is prepared by dissolving 40 grams silver nitrate in 800 ml. of water and then dissolving 20 grams of potassium hydroxide in the solution.

Concentrated ammonia solution is added slowly with vigorous stirring. The brown precipitation formed upon addition of potassium hydroxide to the silver nitrate solution dissolved with the addition of ammonia. Ammonia is added until the solution is completely clear except for a small amount of heavy precipitation at the bottom of the container which Will appear to remain uneflfected by the addition of ammonia. An 8% solution of silver nitrate is added until the solution is slightly cloudy. Precaution: this solution is unstable and should be stored in a brown bottle and discarded after approximately 24 hours.) After the silverizing solution is deposited on the polyethylene, 1 /2 ml. of a reducing solution per square centimeter of polyethylene surface to be coated is added to the bath. (The reducing solution is prepared as follows: 90 grams of granulated sugar is dissolved in 1 liter of water and then 4 ml. of nitric acid is added. The solution is boiled for five minutes, cooled and 157 ml. of ethyl alcohol is added as a preservative.) The bath is agitated for nine minutes after the addition of the reducing solution and then the polyethylene sample is removed from the bath using care to avoid touching the silvered surface. The sample is quickly washed twice with water to remove any smudge from the silvered surface. The silvered surface is then lightly wiped with a wet absorbent cellulose sponge to remove any stains. The sample is thoroughly washed with water.

The silverized porous polyethylene sheet is coated with a thin film of palladium-silver alloy by immersing the sheet in a solution prepared by dissolving 41 milligrams of a 25% silver and 75% palladium alloy in 10 cc. of nitric acid. Ammonium hydroxide is added to the solution to obtain a pH of nine and the volume adjusted to 100 cc. The solution contains 10 milligrams of silver and 31 milligrams of palladium per 120 cc. The plating is carried out at a current density of 7.5 ma./cm. a voltage of 1.9 volts and a temperature of 50 C. The anode was platinum. The deposition which is completed in approximately 80 minutes provides a thin film which exhibits good adhesion to the silverized surface as well as good flexibility. An analysis of the film showed the concentration of palladium to be 32% and silver 68%.

Example 2 A sintered porous nickel matrix having a porosity of 85% and an average pore size of eight microns is coated on one surface with a non-porous film of palladium-silver alloy as follows: 10 grams of a palladium nitrate solution containing three grams of palladium nitrate per 100 cc. of Water and 10 grams of silver nitrate containing three grams of silver nitrate per 100 cc. of water are added to 100 cc. of ammonia hydroxide and filtered. The volume is adjusted to give a concentration of six grams of palladium and silver per liter of solution. The porous nickel sample was immersed in the solution and plating is carried out at a current density of 14 ma./cm. a voltage of 1.9 volts and at a temperature of C. suing a platinum anode. The deposition of a non-porous film having a thickness of 16 microns and possessing good adhesion to the nickel surface is completed in 40 minutes.

In Examples 1 and 2, the sample to be coated can be replaced by a matrix of any plastic polymer material such as polystyrene, Teflon, polyvinyl chloride, polyvinylidene chloride, polyvinyl acetate, polyethylene, etc. which is coated with a conductive metallic layer such as copper, nickel, cadmium, platinum, gold or lead.

In addition, the porous matrix can be a metal such as silver, gold, nickel, copper, zirconium, rhodium, iridium, ruthenium, or cadmium. The proper selection of these materials depends upon the end use of the coated material and is within the ability of one skilled in the art.

The above illustrative examples are given as preferred embodiments of the invention, however, the invention is not to be construed as limited thereby. It is possible to produce still other embodiments without departing from the inventive concept herein disclosed and such embodiments are within the ability of one skilled in the art.

What is claimed is:

1. The method of electrodepositing a non-porous layer of palladium-silver alloy onto an article comprising the steps of (l) forming an ammoniacal nitrate solution of the palladium and silver, said solution having a pH of from about 7.5 to 11, (2) immersing the article to be coated in the solution, and (3) applying an electrical current to the solution and plating out said metals.

2. The method of claim 1 wherein the nitrate solution is formed by dissolving a palladium-silver alloy in nitric acid and adding ammonium hydroxide to render the solution basic.

3. The method of claim 1 wherein the solution is formed by admixing solutions of silver nitrate and palladium nitrate and thereafter adding ammonium hydroxide to render the solution basic.

References Cited in the file of this patent UNITED STATES PATENTS 1,903,860 Gockel Apr. 18, 1933 1,921,941 Powell et al Aug. 8, 1933 2,452,308 Lambras Oct. 26, 1948 OTHER REFERENCES Klochko: Chemical Abstracts, April 20, 1943, page 1935; Metal Finishing, August 1945, page 330. 

1. THE METHOD OF ELECTRODEPOSITING A NON-POROUS LAYER OF PALLADIUM-SILVER ALLOY ONTO AN ARTICLE COMPRISING THE STEPS OF (1) FORMINE AN AMMONIACAL NITRATE SOLUTION OF THE PALLADIUM AND SILVER, SAID SOLUTION HAVING A PH OF FROM ABOUT 7.5 TO
 11. (2) IMMERSINGTHE ARTICLE TO BE COATED IN THE SOLUTION, AND (3) APPLYING AN ELECTRICAL CURRENT TO THE SOLUTION AND PLATING OUT SAID METALS. 